Matthew Buquet, the suspect in the Ricin letter case, pleaded not guilty to a single charge of mailing a letter threatening to kill Federal Judge Fred Van Sickle in federal court Wednesday afternoon.

During his first appearance, Buquet was assigned a federal public defender and his bail hearing was set for next Tuesday at 2 p.m.

If convicted on the charge of sending a threatening letter, Buquet faces up to 10 years in prison and a $250,000 fine.

The American Postal Workers Union first alerted its employees about the Ricin letters a week ago Wednesday. Two letters, addressed to the Spokane Post Office and Federal Judge Fred Van Sickle, were mailed May 14, arrived at the post office May 17 and preliminarily tested positive for Ricin, a highly toxic substance made from castor beans for which there is no known antidote.

The indictment said the letter addressed to Judge Van Sickle contained a threat to kill. While the indictment made no mention of ricin, the FBI confirmed early Wednesday evening that the letters did contain the substance,

This discovery prompted the deployment of a National Guard team to Spokane to investigate and help confirm whether or not Ricin was present on the letters.

At the time of the discovery, the APWU told its employees there was no reason to believe any employees were at risk from handling the suspect letters.

The FBI joined the investigation and, on Saturday, participated in a search of the Osmun Apartment at 1st and Oak in Browne's Addition. During that search a man was taken from the building by FBI agents.

A picture of Buquet matches the description of the man neighbors in a Browne's Addition apartment complex say was escorted from the building.

"He just seemed resigned, he wasn't handcuffed or anything. I don't know if he was a suspect or not but he was the only one wearing Bermuda shorts and everyone else was dressed. Who is to say who he is," said Carl Vincent, who lives across the street from where the search warrant was executed Saturday.

On his Facebook page, Buquet, 37, says he's from Bogota, Colombia, studied at electronics engineering at ITT Technical Institute in Spokane Valley and works in maintenance for ABM Industries. A representative from ABM Industries said that Buquet had not worked for that company for more than a month. A representative from ITT Technical Institute said Buquet never went to school there.

It's not known yet what connection Buquet has with Van Sickle that prompted him to allegedly send a threatening letter to him in the mail. Judge Van Sickle has presided over several high profile cases in recent years, including the Kevin Harpham case and the trial of former Spokane Police Officer Karl Thompson.

In those cases, Harpham entered a guilty plea in Sept. 2011 for planting an improvised explosive device along the Martin Luther King Jr. Unity March route in early 2011 while Thompson was convicted and sentenced to 51 months in prison for excessive force and lying to investigators in the Otto Zehm death investigation.

This isn't the first time he has run afoul of the law. Buquet is a registered Level II Sex Offender, having pleaded guilty to child molestation and convicted on an indecent liberties charge in 1998.